palindrome linked list

list.php

/**
 * Definition for a singly-linked list.
 * class ListNode {
 *     public $val = 0;
 *     public $next = null;
 *     function __construct($val = 0, $next = null) {
 *         $this->val = $val;
 *         $this->next = $next;
 *     }
 * }
 */
class Solution {
    /**
     * @param ListNode $head
     * @return Boolean
     */
    function isPalindrome($head) {
        if ($head == null) {
            return true;
        }
        
        // Traverse the list, building a reversed list
        $fast = $head;
        $slow = $head;
        $reversed = null;
        while ($fast != null) {
            $fast = $fast->next;
            if ($fast != null) {
                $fast = $fast->next;
                $node = new ListNode($slow->val, $reversed);
                $reversed = $node;
            }
            $slow = $slow->next;
        }
        
        while($slow != null) {
            if ($slow->val != $reversed->val) {
                return false;
            }
            $slow = $slow->next;
            $reversed = $reversed->next;
        }
        
        return true;
    }
}